In the semiconductor field, conventionally, a silicon substrate (Si substrate) has been used widely as substrate material. However, in recent years, attention has been paid to a silicon carbide substrate (SiC substrate) having physical properties superior to those of a silicon substrate. A silicon carbide substrate has a characteristic that it has a smaller crystal lattice constant and a larger band gap as compared with a silicon substrate, and a characteristic that it is difficult to etch because it has a small crystal lattice constant, that is, has a strong interatomic bond.
Therefore, as a method of etching a silicon carbide substrate, conventionally, the etching method disclosed in the Japanese Unexamined Patent Application Publication No. 2008-294210 has been suggested, for example. This etching method is configured to sequentially perform a mask forming step in which a silicon dioxide film (SiO2 film) having a mask pattern of a predetermined shape is formed on the surface of the silicon carbide substrate, a first etching step in which the silicon carbide substrate is heated to a temperature between 70 and 100° C. and is plasma etched by a gas mixture of SF6 gas, O2 gas and Ar gas using the silicon dioxide film as mask, and a second etching step in which the silicon carbide substrate is heated to a temperature between 70 and 100° C. and is plasma etched by a gas mixture of Ar gas and O2 gas using the silicon dioxide film as mask.
In the above etching method, the reason why etching is performed in two steps: the first etching step and the second etching step, is that, if only the first etching step is performed, as shown in FIG. 5(b), a trench (sub-trench) H′ is additionally formed near the side wall in the bottom of a hole H or trench H formed by etching and it is therefore not possible to accurately etch the silicon carbide substrate. Therefore, the second etching step is performed to make the bottom of the hole H or trench H flat.
On the other hand, the applicant of the present application has suggested, as an etching method capable of etching a silicon carbide substrate in only one etching step, a method in which the silicon carbide substrate is heated to a temperature between 200 and 400° C. and is plasma etched using SF6 gas or using a gas mixture including SF6 gas and O2 gas (the Japanese Unexamined Patent Application Publication No. 2009-246096).
In this etching method, the main two reasons why the silicon carbide substrate is heated to a temperature between 200 and 400° C. are as follows. That is, the first reason is that atoms constituting a silicon carbide substrate are difficult to react with radicals and ions generated by generation of plasma from SF6 gas before the bonds between the atoms are broken, and the bonds between the atoms are more easily broken and the atoms more easily react with radicals and ions generated by generation of plasma from SF6 gas as the temperature of the silicon carbide substrate increases. That is, a silicon substrate has an interatomic distance of silicon of approximately 0.25 nm and a bond energy of approximately 2 eV, while a silicon carbide substrate has an interatomic distance of silicon carbide of approximately 0.189 nm and a bond energy of approximately 4.5 eV, that is, a silicon carbide substrate has a smaller interatomic distance and a higher bond energy as compared with a silicon substrate, and therefore it is necessary to apply a large amount of energy for etching a silicon carbide substrate. Therefore, a silicon carbide substrate cannot be etched in the same way as a silicon substrate.
Further, the second reason is that when the temperature to which the silicon carbide substrate K is heated is low, as shown in FIG. 5(b), an additional sub-trench H′ is easily formed near the side wall in the bottom of a hole H or trench H formed by etching, and when the temperature to which the silicon carbide substrate K is heated is high, as shown in FIG. 5(a), etching proceeds isotropically and the side wall of a hole H or trench H is easily etched (a bowing shape is easily formed).
Therefore, when the temperature to which the silicon carbide substrate K is heated is set between 200 and 400° C., the silicon carbide substrate K can be efficiently etched even though it has a strong interatomic bond. Further, at the time of etching, a sub-trench H′ is never formed in the bottom of a hole H or trench H, or, even if a sub-trench H′ is formed therein, it is a very small one. Furthermore, the side wall of a hole H or trench H is never etched, or, even if the side wall of a hole H or trench H is etched, it is very slight.
It is noted that the reason why formation of a sub-trench H′ is not preferable is that a hole H or trench H formed by etching is for embedding an insulation film or an electrode therein, and, if a sub-trench H′ is formed, concentration of electric field and stress easily occur at the sub-trench H′ and device reliability is therefore reduced, which causes product defects. On the other hand, the reason why it is not preferable that the side wall of a hole H or trench H is etched is that a cavity easily occurs when embedding an insulation film or an electrode in the hole H or trench H and this results in a non-uniform film thickness of the side wall, which causes product defects, such as causing electric leakage